This invention relates generally to scanning beam velocity modulation (SVM), and more particularly to a circuit arrangement employed therein to control generation of unwanted emissions.
The apparent sharpness of a cathode ray tube picture may be enhanced by modulation of the scanning beam velocity in accordance with a derivative of the display picture video signal. The derivative signal, or SVM signal, may be derived from a luminance component of the video display signal and is employed to produce scanning beam velocity variations. Slowing the scanning velocity of the electron beam results in a localized brightening of the displayed image, whereas acceleration of the scanning velocity results in a localized darkening of the display. Thus, edges of the displayed image may be perceived to have a more rapid transition or faster rise time by varying the intensity of the display about the edge. This method of sharpness enhancement provides various advantages over that provided by video frequency response peaking, for example, blooming of peaked high luminance picture elements is avoided, and in addition, unwanted video noise occurring within the bandwidth of the video peaking arrangement is not enhanced.
The velocity of the scanning beam may be modulated by an SVM coil, positioned on the CRT neck to generate a supplementary or SVM deflection field. The SVM field, in conjunction with the main deflection field, produces electron beam acceleration or deceleration responsive to the polarity of current in the SVM coil. Thus the amount of beam acceleration or deceleration is proportional to the magnitude of the SVM current, which in turn is proportional to the amplitude of image components within the display signal.
Deficiencies in SVM signal processing may generate unwanted products or harmonics which are readily radiated or conducted both within and beyond the display device. However, although careful attention to circuit design and layout may significantly reduce the generation of unwanted SVM products, the actual display signal content may result in of unwanted SVM products, the actual display signal content may result in SVM signals of sufficient magnitude and spectral composition to result in unintentional emissions beyond the display device.
Unintentional emissions are prevented by an inventive arrangement for analysis of display signal content and generation of an amplitude control signal. An apparatus for scanning beam velocity modulation comprises a scanning velocity modulation signal processor for generating an scanning velocity modulation deflection signal responsive to a scanning velocity modulation signal. A generating means supplies a feed forward signal to the processor for controlling the scanning velocity modulation deflection signal amplitude in response to a component of the scanning velocity modulation signal.